Various forms of sound attenuating earcups are known in the prior art for protecting the ear of the wearer from the effect of ambient noise. Most of these devices rely on the rigidity of the material making up the earcup to suppress noise. It is known that the rigidity of the material of which the earcup is formed effectively reduces relatively high frequency sound.
While sound attenuating earcups of the prior art are effective in attenuating relatively high frequency sound, they are not as effective as is desired in respect of relatively low frequency sound.
There have been developed electronic active noise reduction systems which incorporate a microphone, a feedback amplifier and a speaker. As applied to an earcup, the speaker presents the desired signal such as information or music. Noise which penetrates the earcup from the outside interferes with the desired signal. The microphone samples the combined noise and signal and supplies it to the feedback amplifier.
The feedback amplifier receives two inputs, one from the microphone and one from the input to the speaker. The feedback amplifier takes the input to the speaker and uses it to subtract the desired signal from the combined noise and signal so that after the subtraction the output of the amplifier is only the noise. This noise signal is inverted and added to the desired input to the speaker. The inverted noise portion of the speaker output then combines with and cancels the noise which penetrates the earcup. As a result, only the desired signal with no noise interference reaches the user's ear.
Electronic active noise reduction systems such as those described above are effective at low frequencies of less than 2,000 Hz. However, earcups incorporating such electronic active noise reduction systems are inordinately expensive.